Hi!
Now I have some experience on Visionarry with daggerboards and rudders.
WW hull induces drag that is counteracted by negative AOA on the rear
rudder. So when ww-hull is on the left side must helmsman turn steering
wheel to the right to compensate the drag. It looks like lowering rear
daggerboard(asymmetric) reduces the need to compensate this drag with
rear rudder.
I can feel the rudder becoming more neutral when rear daggerboard is lowered
Whether this is caused by increased drag or created lift or both I do
not know. Yet I do not have found out which gives highest speed.
When sailing upwind without jib the daggerboards are mandatory. Now that
the rudders are only 95cm long it is really more difficult to get the
boat sailing from zero speed and daggerboards do help a lot.
By the way I could imagine an Atlantic proa is easier to shunt because
drag does not cause luffing away.
Regards,
Arttu
> OK! I am about to drill down to China. (Hope that expression has the same
> meaning where you're from... I guess not... if you're in China) I hope I
> can explain this without putting you to sleep... and hopefully you can
slap me
> out of it. There are two issues I want you to entertain... efficiency and
> strength. We want to go fast... and we want to stay in one piece. I think
> those are universal. We are "thinking" about a HarryProa (nothing
else really
> matters)...
>
> Efficiency
> If you raise the front rudder, you have to kick the back rudder till
the angle
> of attack (AOA) of the entire hull counters the leeward force of the
sails. We
> all know that low aspect foils are inefficient. And the leeward hull
is about
> as low aspect as you can get. So even though its big and strong,
there is a lot
> of side slip going on. We are aiming for Europe and we hit Africa.
We also
> have a LOT of induced drag.
>
> Now if we lower the forward rudder, we can get the two rudders (acting
as highly
> efficient, high aspect foils) doing all the heavy lifting. We can now
set the
> AOA of the hull to be zero by having positive AOA on the rudders. In
this case,
> the hull is presenting the least drag it can possibly do and the
rudders are
> taking all the load. So we're kicking ass and taking names, until...
>
> Strength
> ... you start realizing that we need to design for the "lifting
windward hull"
> case. As an approximation... the leeward force is about equal to the
windward
> hull's weight. Split that in two (for the two rudders) and for my
> case... that's 4000 lbs (17 kN). (Forgive me Rob... you have the best
> pictures) http://www.harryproa.com/building_Vis/Vis_img_feb11.htm. By
the time
> you get the moment arms into the issue, the bottom rudder post would
be taking
> somewhere around 11,000 lbs (50 kN). I haven't analyzed these, but I think
> these supports would have troubles with that load. Fortunately, they
don't have
> to in Rob's design! If you'll note the rudder isn't balanced so the
lateral
> forces acting on the rudder are resisted... by... you got it... the
helmsman.
> If I were to take a WAG, I'd say Rob has a mechanical advantage
somewhere around
> 100:1 on the wheel. So to hold that, the helmsman would have to
supply... 100
> lbs (500 N) On each wheel... not bloodly likely. So, Rob has used the
helmsman
> as a force limiting transducer. Briliant!
>
> Before I saw Rob's elegant brilliance, I was going to make my rudders
> (near) balanced. I wanted some helm feel, but I didn't want a work
out or blow
> the autopilot. Then I realized using delicate members like Rob, I'd
soon tear
> them off while sipping my afternoon tea and steering with one finger.
>
>
> Can anyone... tell me how I can have my rocket ship and eat it too?